This study investigated cell-mediated immune responses in Lyme disease in the rhesus macaque model. We focused on the cellular responses to p39 of B. burgdorferi, a highly immunogenic lipoprotein putatively localized to the spirochete surface. Previous studies in our lab showed that peripheral blood mononuclear cells (PBMCs) from tick-inoculated rhesus respond marginally (4<SI<17) in vitro to whole heat-killed spirochetes between weeks 2-8 post-inoculation (PI) and maximally (25<SI<48) between weeks 24-52 PI. However, with time, the antigen response is significantly down-regulated. To understand the mechanism of unresponsiveness to antigen in vitro, T-cell proliferative responses to p39 and heat-killed spirochetes were assessed, as a function of time, in rhesus monkeys with late and early B. burgdorferi infections. T-cell blastogenesis to antigens was conducted weeks 162, 169 and 183 PI in animals with late infection, and weeks 0, 2 and 15 PI in animals with early infection. T-cells from animals with late infection did not respond to heat-killed spirochetes or the p39 antigen. Surprisingly, however, T-cells from animals with early infection also did not respond to both antigens. To determine the role of cytokines in the T-cell unresponsiveness to antigens, exogenous interleukin (IL)-2 and IL-12 were added to antigen cultures in an attempt to reverse the response to antigens. IL-2 did not enhance the antigen response in any animals whereas, IL-12 partially reversed the cellular response to heat-killed spirochetes in three animals, suggesting that cytokines other than IL-2 and IL-12, are probably involved in regulating cellular responses to heat-killed spirochetes and the p39 antigen in vitro. The findings from this study show that B. burgdorferi spirochetes can induce a specific down-regulation of the blastogenic response in a cohort of animals. The immunological basis for this phenomenon is the focus of future studies in the rhesus macaque model.